This invention relates to an adaptive filter for use in identifying an unknown system. It is to be noted here that such an adaptive filter is used as an echo canceller for cancelling an echo which occurs in a two-wire/four-wire hybrid, an equalizer for cancelling an intersymbol interference imposed on a transmission line, a noise canceller for cancelling a noise leaking in an acoustic microphone, a howling canceller for cancelling howling which results from acoustic coupling between a loudspeaker and a microphone, and so on.
A conventional adaptive filter of the type described is coupled to an unknown system to be identified. Herein, the unknown system may be, for example, a two-wire/four-wire hybrid, a transmission path, and the like. At any rate, the adaptive filter is supplied from an external device, such as a microphone, with an input signal given in the form of a sequence of sampled input signals each of which is sampled at every one of sampled periods. In addition, the adaptive filter is supplied with an external signal which is sent from the unknown system and which may be, for example, an echo signal in an echo canceller.
At any rate, the adaptive filter produces, as an output signal, an identification error signal appearing as a result of identification of the unknown system. The identification error signal may be simply called an error signal hereinunder. In order to obtain the error signal, the adaptive filter calculates a replica of the echo signal from the input signal and the error signal to produce an echo replica signal representative of the replica of the echo signal and subtracts the echo replica signal from the echo signal to produce the error signal.
More specifically, the adaptive filter comprises a plurality of delay elements which define taps and which are connected in cascade to one another through the taps. The sampled input signals are successively given to the delay elements on one hand and given to the unknown system on the other hand. The sampled input signals are delayed by the sample periods to be produced as delayed signals through the taps while they are produced through the unknown system as the external signal. The taps are connected to a plurality of tap gain control circuits which determine tap coefficients of the taps. The tap gain control circuits produce tap coefficient signals representative of the tap coefficients.
With this structure, the gain control circuits supply the tap coefficient signals to an adder circuit to calculate a sum of the tap coefficient signals. The adder circuit delivers, as the echo replica signal, a sum signal representative of the sum to a subtractor supplied with the external signal. The subtractor subtracts the echo replica signal from the external or the echo signal to produce the error signal. Furthermore, the error signal is delivered to the tap gain control circuits to adaptively renew or modify the tap coefficients of the taps.
In order to renew or modify the tap coefficients, proposal has been made about adaptive filters which are operable in accordance with LMS (least mean square) algorithm which is described in "Adaptive Signal Processing" published in 1985 by Prentice Hall and a learning identification method (LIM) which is disclosed in IEEE Transactions on Automatic Control (Vol. 12, 3, 1967 on pages 282 to 287), respectively. In any event, such adaptive filters should have taps of a number determined by a length of an impulse response in the unknown system. In other words, the number of the taps increases as the impulse response becomes long.
In the interim, it often happens that the impulse response has a long flat delay portion, a significant part of a dispersive portion, and a tail portion in a certain unknown system, such as a satellite communication system. An adaptive filter may be used as an echo canceller to cancel an echo in the satellite communication system. Inasmuch as the impulse response has long flat delay part and taps must be prepared for such a long flat delay part also, the adaptive filter should have a great number of the taps.
Herein, it is pointed out that the tap coefficients for the long flat delay part might be equal to zero and actually become useless on calculation of the echo replica signal. Taking this into consideration, an identification method has been proposed in an article contributed by S. Kawamura et al to ICASSP 86 Tokyo (CH2243-4/86/0000-2979 1986, 1986) (pages 2979 to 2982) so as to effectively identify an unknown system even when an impulse response has a long flat delay part together with a significant part. With this method, processing is made by selecting tap coefficients which are positioned at the significant part and by calculating the echo replica signal by the use of only the selected tap coefficients. Such selected tap coefficients are successively changed from one to another until an optimum tap set is found out.
However, a long time is required in the above-mentioned method to approximate the impulse response having the long flat delay part and to converge the tap coefficients into stable values.